1. Field of the Invention
This invention relates to an installation for the continuous heat treatment of wire rod.
2. Description of the Prior Art
Wire rod is, of course, a hot-rolled steel semi-product with numerous possibilities of use. The mechanical properties required of wire rod differ according to the intended area of application. In this respect, wire rod is divided up into hard wire and soft wire. This distinction is based both on the steel carbon content and its breaking strength. Soft wire frequently has a carbon content below 0.4%, and preferably below 0.15%, and the lowest possible breaking strength, e.g. less than approximately 400 MPa, while hard wire contains at least 0.4% of carbon and the highest possible breaking strength. The breaking strength of wire rod depends largely on the cooling applied to the wire.
Various cooling treatments for wire rod known today comprise immersing it in a water bath at a temperature of at least 75.degree. C., these treatments being applied immediately after the hot-rolling mill and being intended for either soft wire or hard wire. In these processes, the wire rod is usually disposed in non-contiguous turns spread out on a conveyor. In a cross-section of the conveyor the temperature distribution in the layer of turns is not uniform. Because of the difference in the metallic mass between the center and the sides, the center of the layer is automatically less hot than the sides, which comprise wire in the form of coils as it were.
It is advantageous to recall the above known points, because they have an effect on the conduct of the wire rod treatment processes.
For example, in order to obtain required strength properties, it is known that the hard wire must undergo at least 90% of its allotropic transformation .gamma..fwdarw..alpha. while being immersed in the hot water bath. That means that it must enter the bath with its least hot part (the center) at 720.degree. C. at least and leave this bath with its hottest part (sides) at above 500.degree. C.
On the other hand, the soft wire must undergo at least 80%, and preferably at least 95%, of its allotropic transformation .gamma..fwdarw..alpha. before entering the hot water bath. When the wire enters the water bath, its hottest part (the sides) must therefore be at a temperature such that the allotropic transformation is substantially completed throughout the wire. Also, when the wire leaves the water bath, the temperature of its coldest part (the center) will preferably be at least 520.degree. C.; this temperature must be sufficient to comply with a temperature of at least 475.degree. C. to re-form the coils. In the latter case, cooling in a hot water bath does not involve any metallurgical effect; it basically allows the cooling time to be reduced.
The use of these different treatments makes it necessary to use appropriate installations for soft wire and hard wire either in the form of separate installations or in the form of a single convertible installation.
This invention relates to a processing installation of the convertible type.
Known installations of this type usually comprise a stationary water bath, an outlet conveyor situated downstream of the water bath, and an inlet conveyor situated upstream of the water bath. The inlet conveyor is so dimensioned as to allow the soft wire to cool before it enters the bath. The considerable length of this inlet conveyor makes it difficult in the case of hard wire, to maintain an adequate temperature until the wire enters the bath.
The space occupied by the installation, however, must take into account the greatest length required of the inlet conveyor.